Wireless communications networks (WCN) manage mobility of a wireless mobile device by collecting radio information about the network. From the advent of location-based services, this radio information has been used to provide low and medium accuracy location estimates.
In non-soft handoff systems, the location of every active mobile in the network is known to the nearest serving cell and sector. The identification of the serving cell and serving sector can be converted to a location estimate by simple translation to a pre-established latitude and longitude for the serving cell and/or sector.
Inclusion of the WCN measured time or mobile measured power based range estimate from the serving cell to the mobile position provides a method for refining the basic serving cell identifier based location estimate with minimal additional calculations.
A further refinement of the cell/sector identifier plus ranging method using the mobile-collected network information from one or more potential handover neighboring cells is generally known as Enhanced Cell-ID (ECID). The ECID technique relies on the mobile unit's ability to record the power levels from the beacons (also known as pilots) of multiple potential handover candidate/neighbor cells. This technique adds absolute power based and/or power-difference-of-arrival (PDOA) based measurements to improve the serving cell ranging location estimate.
Since typically the received signal power of various nearby transmitting cell sectors measured by the active mobile device is known by the WCN, the PDOA for ECID value is based on the received signal levels measured by the mobile for the serving cell's and/or one or more potential handover candidate/neighboring cell's beacons. Since the PDOA data collection requires visibility to two or more neighbor cell sites, location yield will be less than 100%. The effects of RF multipath, mobile receiver quality, and granularity of the measurement all act to reduce location accuracy for ECID.
ECID in GSM, UMTS and LTE
In GSM, ECID is also known as Network Measurement Report (NMR) location. The NMR is generated by the mobile to provide the WCN with information regarding the serving and neighboring cells to facilitate handover as described in GSM/3GPP Technical Standard 05.08, “Radio subsystem link control” section 3 (Handover).
The Enhanced Cell ID positioning technique is standardized as “Timing Advance” positioning in 3GPP TS 43.059 , “Functional stage 2 description of Location Services (LCS) in GERAN” section, section 4.2.1. In LTE networks the “enhanced cell ID method” is described in 3GPP TS 36.305, “Stage 2 functional specification of User Equipment (UE) positioning in E-UTRAN” Section 4.3.3.
In the example GSM system, the NMR contains the mobile generated Measurement Results. The purpose of the Measurement Results information element is to provide the results of the measurements made by the mobile station regarding the serving cell and the neighbor cells. The Measurement Results information element is coded as shown in GSM/3GPP Technical Specification 04.08, “Mobile radio interface layer 3 specification” section 10.5.2.20 (Measurement Report).
The mobile location center (MLC) uses NMR delivered serving cell-id (in GSM the Cell-Global-Identity (CGI) gives the cell and sector) to consider the cell site's geographical location as the reference point. The reported timing advance (TA) value of the current serving cell allows computation of the range from the reference point. The Received Signal Strength Indicator (RSSI) of the serving cell is corrected with the current mobile dynamic power control settings, when received on traffic control channel instead of broadcast control channel. The corrected RSSI value of serving cell is then normalized with its known value of broadcast effective radiated power (ERP). The Reception Level (RxLev) values of the reported neighboring cells over Broadcast Control Channel (BCCH) beacons are then normalized against their known value of broadcast effective radiated power (ERP). Using the serving cell's antenna position, the TA-derived range, and the PDOA from three or more sites, a location estimate can be calculated.
Since ECID can use PDOA multi-lateration, the geographic layout of the neighbor cells also affects the quality of the location through geographic dilution of precision (GDOP). The limitation of only up to six neighbor cell RxLev measurements present in the NMR limits accuracy, when NMR data is not collected over a sufficient amount of the time interval by limiting potential GDOP reduction though receiver site selection.
Since the PDOA measurement requires averaging over multiple samples to nullify the received signal fast fading effects (the GSM NMR is transmitted by the mobile station periodically during an active call), latency is much higher than for other cell-ID based techniques.
Since the RSSI measurement for only the serving cell, when the mobile is in active mode is based on the variable power settings for the BTS, normalization of the serving cell RSSI before inclusion into the PDOA calculation requires knowledge of the BTS forward (downlink) power control settings from the GSM WCN.
Calibration may be used improve accuracy in ECID location systems. ECID Calibration can include the use of predictive RF propagation mapping and extensive drive testing to create a grid of CGI/RxLev “fingerprints”. By mapping the neighbor list and received signal levels over the coverage area, it is possible to achieve medium accuracy results within the range of 200-500 meters in networks having relatively high BTS density.
In U.S. Pat. No. 7,434,233, a single site ECID location system is taught where the power measurements from a single 3-sector Base Transceiver Station (BTS) with a serving sector and two co-sited sectors allow the formation of a sector limited timing range band and a directional angle from the BTS cell site.
The inventive techniques and concepts described herein apply to time and frequency division multiplexed (TDMA/FDMA) radio communications systems including the widely used IS-136 (TDMA), GSM, and Orthogonal Frequency Division Multiplexed (OFDM) wireless systems such as LTE, LTE-Advanced and IEEE 802.16 (WiMAN/WiMAX). The Global System for Mobile Communications (GSM) model discussed is an exemplary but not exclusive environment in which the present invention may be used.